Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
One result of the above is the emergence of micro-electrical-mechanical-systems (MEMS) based transducer devices. These may be for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. There is also a continual drive to reduce the size and cost of these devices.
Microphone devices formed using MEMS fabrication processes typically comprise a membrane with electrodes for read-out/drive deposited on the membrane and a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of transducers, the device is driven by a potential difference provided across the electrodes.
FIG. 1a shows a basic MEMS device 1 comprising a substrate 3 having a membrane 5 formed thereon. The substrate 3 comprises a back-volume 7. The back-volume 7 is formed using an etching process from below the substrate, known as a “back-etch”. The back-volume 7 forms an important part of a MEMS device, since the back-volume enables the membrane to move freely in response to incident sound or pressure waves.
The substrate has a width “X” and a height “Y”. For example, the width X may be typically 1.5 mm, and the depth Y typically 625 μm. The diameter of the membrane 5 is typically 1 mm.
Although not shown in FIG. 1a, it will be appreciated that, in order to incorporate the transducers into useful devices, it is necessary to interface or couple them to electronic circuitry (not shown), which may either be located on the same substrate or a separate integrated circuit.
FIG. 1b shows a view of the MEMS device 1 from underneath the substrate 3, having a back-volume 7 etched therein. The back-volume has a diameter of typically 900 μm.
There is a continual drive to reduce the overall size of a MEMS device 1, particularly when such devices are to be incorporated into portable electronic equipment. However, as will be appreciated, reducing the size, and in particular the height, of the MEMS device has the consequential effect of reducing the size and hence volume of the back-volume 7. That is, an obvious method of reducing the height of the device is to reduce the thickness of the substrate 3, and this will cause the back-volume 7 to reduce in size also. Reducing the size of the back-volume 7 can have a degrading effect on the output signals produced by the MEMS device 1. It will therefore be appreciated that a trade-off exists between the size and performance of the MEMS device.
This is because the back-volume 7 must be of sufficient size to produce sufficient compliance, i.e. compression, to allow a substantially un-damped movement, i.e. deflection, of the membrane. In a microphone having a small back-volume the compliance is reduced and therefore the sensitivity is reduced. Pressure relief holes (not shown) are required between the back-volume 7 and the atmosphere to prevent pre-stressing of the membrane. These pressure relief holes introduce an acoustic impedance between the back-volume and the atmosphere which, in relationship with the compliance of the back-volume, introduce a 1/f noise spectrum into the microphone output.
Increasing the back-volume increases the signal-to-noise ratio (SNR) of the microphone. The larger the back-volume becomes, the greater the compliance, i.e. the less the impedance, of the back-volume becomes. As a consequence, the lower in frequency the 1/f noise spectrum, due to the pressure relief holes, becomes.
One way of overcoming the drawback of reducing the back-volume when reducing the height of the MEMS device is to increase the diameter or area of the back-volume 7, such that a reduction in height is offset by the increased diameter or area. However, the amount by which the diameter or area of the back-volume 7 can be increased is limited by the diameter of the membrane. For example, with the dimensions given as examples in FIGS. 1a and 1b, the area of the back-volume 7 cannot be increased significantly above 900 μm, since the diameter of the membrane is only 1 mm.
It is therefore an aim of the present invention to provide a MEMS device that is capable of increasing the back-volume for any given height.